The present invention relates to a displacement drive with an obstruction prevention function for movable parts such as, for example, windows and sunroofs in motor vehicles.
In a displacement drive of this kind known from German Published Patent Application No. 37 36 400 A1, a decrease in the rotation speed or velocity of the drive is performed in order to achieve a predefined spring rate in specific displacement ranges so as to lower the kinetic energy. The movable part is moved by way of a drive motor, through an obstruction hazard range in which foreign objects may become jammed between the movable part and a stationary part, into a closed position. Because of unavoidable delays in signal processing and the inertia of the mechanically moved part, the foreign object can be jammed between the moved part and the stationary part despite a safety apparatus representing the obstruction prevention function. To prevent this, provision is made in the case of this known displacement drive for the displacement velocity during closing of the movable part to be diminished in the obstruction hazard range. The actual obstruction prevention function can then, if applicable, halt the movable part momentarily and possibly reverse the direction of motion.
With this known displacement drive, the decrease in velocity takes place in stages, i.e. operation is switched in one step from a high stage to a low stage of the closing velocity. Switching down abruptly in this fashion can cause the obstruction prevention function to respond incorrectly, and moreover lengthens the closing time.